Economy & Politics

Non-statistics “90 percent effective” – clarification of a misunderstanding

Two corona vaccines could soon be approved
Two corona vaccines could soon be approvedimago images / Hans Lucas

The results of the corona vaccine studies sound promising. In early November, the biotech company Biontech and the pharmaceutical giant Pfizer reported that their active ingredient was “90 percent effective”. In mid-November, the partners corrected their statement again upwards to 95 percent. Other vaccine developers also say that their vaccine is up to 95 percent effective.

But to whom does this value apply. The Bavarian Broadcasting Corporation said at the beginning of November: “That means 9 out of 10 people can be protected from infection by the vaccination”. The Berliner Zeitung also writes: “An efficiency – or effectiveness – of 90 percent means that 9 out of 10 people can be protected by vaccination from Covid-19.” According to this, around 90 percent of people vaccinated would be prevented from being infected with the virus for sure.

However, this is not true, as the non-statistics make clear. The vaccine developers do not report effectiveness for the people who have been vaccinated. Instead, they refer to those infected with corona, write the unstatistics experts.

Effectiveness in infected people instead of vaccinated people

According to the Pfizer study protocol, the effectiveness is defined as follows: The starting point is two groups, one receives the corona vaccine, a second receives a placebo. According to this, the proportion of Covid-19 cases among the vaccinated study participants is divided by the proportion of cases in the control group. The result is subtracted from 1 and multiplied by a hundred.

With a total of 43,000 study participants at Biontech / Pfizer, this means that for 90 percent effectiveness there would have to be eight cases among the vaccinated and 86 cases in the placebo group. With an effectiveness of 95 percent, the ratio would be eight cases in the vaccine group and 156 in the placebo group.

This calculation does not refer to the number of people vaccinated or all study participants, but to the number of people infected. It is therefore a relative risk reduction – and not an absolute one. To illustrate this, the non-statisticians take the flu shot as an example. According to this, the effectiveness of the flu vaccination in a season with low prevalence is around 50 percent. This means that for every 100 people without a vaccination, two will become infected with the flu. In contrast, for every 100 people with vaccination, only one person becomes infected.

In addition, the effectiveness does not allow any statements about serious illnesses and deaths, according to the statistic. “We can only hope that this reduction will affect serious illnesses to the same extent,” they say. “But that is not being investigated in the current studies.”


With the “Unstatistics of the month” The Berlin psychologist Gerd Gigerenzer, the Dortmund statistician Walter Krämer and RWI Vice President Thomas Bauer question both recently published figures and their interpretations every month.
You can find all “non-statistics” on the Internet at www.unstatistik.de. The book “Why fat doesn’t make you stupid and GM maize doesn’t kill – About the risks and side effects of unstatistics” is published by Campus Verlag.


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